On Fri, Jul 27, 2018 at 05:02:37PM +0100, Robin Murphy wrote:

On 27/07/18 15:10, Dmitry Osipenko wrote:

quoted

On Friday, 27 July 2018 12:03:28 MSK Will Deacon wrote:

quoted

On Fri, Jul 27, 2018 at 10:25:13AM +0200, Joerg Roedel wrote:

quoted

On Fri, Jul 27, 2018 at 02:16:18AM +0300, Dmitry Osipenko wrote:

quoted

The proposed solution adds a new option to the base device driver
structure that allows device drivers to explicitly convey to the drivers
core that the implicit IOMMU backing for devices must not happen.

Why is IOMMU mapping a problem for the Tegra GPU driver?

If we add something like this then it should not be the choice of the
device driver, but of the user and/or the firmware.

Agreed, and it would still need somebody to configure an identity domain so
that transactions aren't aborted immediately. We currently allow the
identity domain to be used by default via a command-line option, so I guess
we'd need a way for firmware to request that on a per-device basis.

The IOMMU mapping itself is not a problem, the problem is the management of
the IOMMU. For Tegra we don't want anything to intrude into the IOMMU
activities because:

1) GPU HW require additional configuration for the IOMMU usage and dumb
mapping of the allocations simply doesn't work.

Generally, that's already handled by the DRM drivers allocating
their own unmanaged domains. The only problem we really need to
solve in that regard is that currently the device DMA ops don't get
updated when moving away from the managed domain. That's been OK for
the VFIO case where the device is bound to a different driver which
we know won't make any explicit DMA API calls, but for the more
general case of IOMMU-aware drivers we could certainly do with a bit
of cooperation between the IOMMU API, DMA API, and arch code to
update the DMA ops dynamically to cope with intermediate subsystems
making DMA API calls on behalf of devices they don't know the
intimate details of.

quoted

2) Older Tegra generations have a limited resource and capabilities in regards
to IOMMU usage, allocating IOMMU domain per-device is just impossible for
example.

3) HW performs context switches and so particular allocations have to be
assigned to a particular contexts IOMMU domain.

I understand Qualcomm SoCs have a similar thing too, and AFAICS that
case just doesn't fit into the current API model at all. We need the
IOMMU driver to somehow know about the specific details of which
devices have magic associations with specific contexts, and we
almost certainly need a more expressive interface than
iommu_domain_alloc() to have any hope of reliable results.

This is correct for Qualcomm GPUs - The GPU hardware context switching 
requires a specific context and there are some restrictions around
secure contexts as well.

We don't really care if the DMA attaches to a context just as long as it
doesn't attach to the one(s) we care about. Perhaps a "valid context" mask would
work in from the DT or the device struct to give the subsystems a clue as to
which domains they were allowed to use. I recognize that there isn't a
one-size-fits-all solution to this problem so I'm open to different ideas.

Jordan

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